@unknownuser said:

Window>Preferences>OpenGL

That seems to be working. My selected-area-hatching is back.
Thanks a lot, you too Gaieus, I checked but face style was set to blue.

Selecting an area gives the selected area this (default blue) hatch. Working in Architectural Style or the Default Style the selected-area hatch doesn't appear. I tried other Styles but a single time using a Sketchy Style the hatch re-appeared. Switching back to Architectural the hatch stayed for the first several selected areas and then disappeared again.
Selecting an area works, because when clicking an area and then delete it is gone. But this is about the hatch that indicates I selected an area.
Can someone help me out please?